Prevention of flooding of fuel cell stack

ABSTRACT

A fuel cell stack ( 1 ) generates power by an electrochemical reaction between hydrogen and oxygen in plural stacked fuel cells ( 2   a   , 2   b ). Each fuel cell ( 2   a   , 2   b ) comprises an anode (26 a ) to which hydrogen is supplied, a cathode ( 26   b ) to which air containing oxygen is supplied, and a electrolyte membrane ( 20 ) which conducts hydrogen ions from the anode ( 26   a ) to the cathode ( 26   b ). The fuel cells ( 2   a   , 2   b ) comprise center cells ( 2   a ) and end cells ( 2   b ). By arranging the moisture absorption capacity of the end cells ( 2   b ) to be larger than that of the center cells ( 2   a ), flooding in the end cells ( 2   b ) which do not easily rise in temperature is prevented, and the low-temperature start-up performance of the fuel cell stack ( 1 ) is enhanced.

RELATED APPLICATION

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/JP2004/006679, filed on May 12, 2004,which in turn claims the benefit of Japanese Application No.2003-136791, filed on May 15, 2003, the disclosure of which Applicationsare incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to prevention of flooding of a fuel cell stack.

BACKGROUND OF THE INVENTION

In a fuel cell stack which is a stack of polymer electrolyte fuel cells(PEFC), hydrogen gas supplied to an anode is ionized, conducts on apolymer electrolyte membrane to reach a cathode as proton, and reactswith oxygen supplied to the cathode and electrons to form water. Amongthe reaction enthalpies between hydrogen and oxygen, energy which is notused for generating power is converted into heat.

JP2002-208421A published by the Japanese Patent Office in 2002,discloses prevention of the moisture generated in the fuel cells fromfreezing by evacuating the fuel cells using dry gas or low humidity gaswhen the fuel cell stack stops operation at low temperature. This freezeprevention is particularly needed in a fuel cell stack for vehicles inwhich running conditions change sharply.

SUMMARY OF THE INVENTION

When a fuel cell stack which has stopped starts again, when warmup isperformed by the heat produced by the power generation reaction,moisture is produced in the fuel cell stack again. Although thisproduced moisture is absorbed by the electrolyte membrane or diffuses tothe anode via the electrolyte membrane, if the moisture amount exceedsthe moisture absorption capacity of the electrolyte membrane, moisturewill overflow to the cathode. This phenomenon is called flooding.

When the temperature during startup of the fuel cell stack is very low,the moisture which overflowed to the cathode freezes despite heatgeneration due to the power generation reaction of the fuel cell. Thefrozen moisture covers the cathode surface, prevents cathode gas fromreaching the cathode, and interferes with the power generation reaction.Therefore, it may be difficult to start the fuel cell stack even ifmoisture had previously been removed by the prior art technique.

In a fuel cell stack which is starting up, the cells at the ends tend tobe more difficult to warm up than cells in a center position. This isbecause heat tends to escape from the ends where there is a largecontact area with the open air. Therefore, when starting the fuel cellstack below freezing point, the time until the ends of the fuel cellrise above freezing point is longer than the time until the centerposition rises above freezing point. As a result, the possibility thatflooding will occur below freezing point is larger at the ends than incells in a center position.

It is therefore an object of this invention to prevent flooding duringstartup by taking the startup characteristics of the fuel cell stackinto consideration.

In order to achieve the above object, this invention provides a fuelcell system comprising a fuel cell stack comprising a plurality of fuelcells stacked in series. The fuel cells comprise a first fuel celldisposed in a center position of the fuel cell stack with respect to astacking direction of the fuel cells, and a second fuel cell disposed ina position other than the center position. The second fuel cell isarranged to have a larger moisture absorption capacity than the firstfuel cell.

This invention also provides a fuel cell stack generating electric powerthrough electrochemical reaction of hydrogen and oxygen. The fuel cellstack comprises a plurality of fuel cells stacked in series. Each of thefuel cells comprises an anode to which hydrogen is supplied, a cathodeto which air containing oxygen is supplied and an electrolyte membranewhich conducts hydrogen ions from the anode to the cathode. The fuelcells comprise a first cell disposed in a center position of the fuelcell stack with respect to a stacking direction of the fuel cells, and asecond cell disposed in a position other than the center position. Thesecond cell is arranged to have a larger moisture absorption capacitythan the first cell.

This invention also provides a fuel cell stack which generates power byan electrochemical reaction between hydrogen and oxygen. The fuel cellstack comprises a plurality of fuel cells stacked in series. Each of thefuel cells comprises an electrode and a gas passage facing theelectrode. The fuel cells comprise a first cell disposed in a centerposition of the fuel cell stack in the stacking direction of the fuelcells, and a second cell disposed in a position other than the firstcell, and the gas passage of the second cell has a largercross-sectional area than the gas passage of the first cell.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell system according to thisinvention.

FIG. 2 is a schematic diagram of a unit cell according to thisinvention.

FIG. 3 is a flow chart describing a routine for stopping the fuel cellsystem performed by a controller according to this invention.

FIG. 4 is a flow chart describing a routine for starting up the fuelcell system performed by the controller according to this invention.

FIGS. 5A-5F are timing charts describing a variation of the outputvoltage and output current of a fuel cell, and the temperature of a fuelcell stack, during startup of the fuel cell system.

FIG. 6 is a schematic diagram of a fuel cell system according to asecond embodiment of this invention.

FIG. 7 is a flow chart describing a routine for stopping the fuel cellsystem performed by the controller according to second embodiment ofthis invention.

FIG. 8 is a schematic diagram of a fuel cell system according to a thirdembodiment of this invention.

FIG. 9 is a flow chart describing a power supply start routine of thefuel cell system performed by the controller according to the thirdembodiment of this invention.

FIG. 10 is a flow chart describing a flooding prevention routineperformed by the controller according to the third embodiment of thisinvention during steady-state operation of the fuel cell system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a fuel cell system for a vehicleaccording to this invention comprises a fuel cell stack 1, hydrogensupply passage 14, air supply passage 15, humidifiers 6, 7 and acombustor 19. The fuel cell stack 1 is electrically connected to anelectrical load 10 via a constant current control circuit 8 and aninverter 9. The inverter 9 converts direct current output by a constantcurrent control circuit 8 into alternating current, and supplies it tothe electrical load 10.

Hydrogen stored in a hydrogen tank or reformed by a reformer is suppliedto the hydrogen supply passage 14. The humidifier 6 humidifies thehydrogen in the hydrogen supply passage 14. Air from a pump or acompressor is supplied to the air supply passage 15. The humidifier 7humidifies air in the air supply passage 15.

The humidifiers 6, 7 have the function of humidifying the circulatinggas to a desired humidity including a relative humidity of zero percent.A bubbler or a steamer can be used as the humidifiers 6 and 7.

The fuel cell stack 1 comprises thirty fuel cells stacked in series.Among the thirty fuel cells, cells 2 a situated in a center position inthe stacking direction are provided with the usual membrane electrodeassembly (MEA) 3. End cells 2 b situated on both sides of the centercells 2 a in the stacking direction are provided with an MEA 3 of highermoisture absorption capacity than that of the center cells 2 a.

Herein, twenty fuel cells situated in the center in the stackingdirection form the center cells 2 a, and five fuel cells respectivelysituated on both sides form the end cells 2 b. The center cells 2 acorrespond to the first cell in the claims, and the end cells 2 bcorrespond to the second cell in the claims.

Each end of the fuel cell stack 1 is in contact with a stainless steelend plate 5 via a heat insulating layer 4. The fuel cell stack 1 istightened in the stacking direction by a long screw which passes throughthe fuel cells 2 a, 2 b, the heat insulating layer 4 and the end plates5. The heat insulating layer 4 comprises an electrically conductingplate member having numerous holes with air sealed inside them.

Referring to FIG. 2, each fuel cell 2 a (2 b) comprises the MEA 3, and apair of separators 23 formed of electrically conducting plate memberswhich are made of carbon graphite. It is also possible to construct theseparators 23 with the electrically conducting plate members which areused for the heat insulation part 4.

The MEA 3 comprises a solid electrolyte membrane 20 and electrodes 26 a,26 b formed on both sides. The electrodes 26 a, 26 b comprise acatalytic layer 21 comprising a platinum (Pt) catalyst supported oncarbon black, and a gas diffusion layer 22 comprising carbon paper.Herein, the electrode 26 a is the anode and the electrode 26 b is thecathode.

In order to increase the moisture absorption capacity, the thickness ofthe membrane 20 of the end cells 2 b is made larger than the thicknessof the membrane 20 of the center cells 2 a. Likewise, the layerthickness of the electrodes 26 a, 26 b of the end cells 2 b is madelarger than the layer thickness of the electrodes 26 a, 26 b of thecenter cells 2 a. This is materialized by making the thickness of thecatalyst layers 21 of the end cells 2 b lager than those of the centercells 2 a, or making the thickness of the 22 gas diffusion layers 22 ofthe end cells 2 b lager than those of the center cells 2 a.

The electrode 26 is formed by mixing a platinum catalyst with Nafion® (®denotes a Registered Trademark) and pure water, stirring, and applyingto carbon paper. The electrode 26 is formed in one piece with the solidelectrolyte membrane 20 by applying a hot press to the solid electrolytemembrane 20 at a temperature of about 120 degrees Centigrade under apressure of about 100 kilogram per square centimeter (kg/cm²) for oneminute. The MEA 3 comprises the electrodes 26 formed in one piece withboth sides of the solid electrolyte membrane 20 in this way. Theelectrode area of the MEA is 300 square centimeters (cm²).

A passage 24 which circulates hydrogen is formed in the separator 23which is in contact with the anode 26 a. A passage 25 which circulatesair is formed in the separator 23 which is in contact with the cathode26 b. Hydrogen humidified by the humidifier 6 from the hydrogen supplypassage 14 is supplied to the passage 24. Air humidified by thehumidifier 7 from the air supply passage 15 is supplied to the passage25. The fuel cells 2 a, 2 b generate electricity by an electrochemicalreaction between hydrogen supplied to the anode 26 a and oxygen in theair supplied to the cathode 26 b, via the solid electrolyte membrane 20.The gas after the reaction at the anode 26 a is discharged as anodeeffluent. After burning the anode effluent with the combustor 19, it isdischarged into the atmosphere. The gas after the reaction at thecathode 26 b is discharged into the atmosphere as cathode effluent.

Next, the method of calculating the moisture absorption capacity of theend cells 2 b will be described.

First, the calorific capacity of the fuel cell is calculated.

For the solid electrolyte membrane 20, NAFION 111 (Dupont®) made fromperfluorocarbon sulfonic acid is used. The specification of NAFION 111is membrane thickness 30 microns (μm), specific heat 0.86 Joules pergram-absolute temperature (J/g-K) and density 1.8 grams/cubic centimeter(g/cm³).

For the gas diffusion layer 22, carbon paper of thickness 300 μm,specific heat 0.86 J/g-K and density 0.2 g/cm³is used. The carbon paperhas numerous pores and has the function of absorbing moisture in thepores.

As the separator 23, carbon graphite of thickness 4 millimeters (mm),specific heat 0.75 J/g-K and density 1.75 g/cm³ is used.

The calorific capacity of the solid electrolyte membrane 20 is 0.0046Joules/absolute temperature-square centimeter (J/K-cm²), the calorificcapacity of the gas diffusion layer 22 is 0.005 J/K-cm² and thecalorific capacity of the separator 23 is 0.53 J/K-cm². The calorificcapacity of the gas diffusion layer 22 is effectively the calorificcapacity of the electrodes 26 a, 26 b. The calorific capacity of onefuel cell which comprises the solid electrolyte membrane 20, the twoelectrodes 26 a, 26 b and the two separators 23 is about 1.1 J/K-cm².

As is clear from the above calculation, most of the calorific capacityis due to the calorific capacity of the separator 23, and it is almostunaffected by the composition and thickness of the solid electrolytemembrane 20, or the thickness of the electrodes 26 a, 26 b.

According to calculation, in the fuel cell stack using this fuel cell,the thermal power required to increase the stack temperature from minus20 degrees Centigrade to 0 degrees Centigrade is about 22 J/cm² per fuelcell. During power generation, in addition to the heat generation by thecell itself, heat is transmitted to the center cells 2 a of the fuelcell stack also from surrounding cells. On the other hand, in the cells2 b at the ends of the fuel cell stack 1, as the heat transmitted fromthe center cells 2 a and the heat emitted from the ends cancel eachother out, a temperature rise due to heat propagation cannot beexpected, and the temperature increases due only to the heat generatedby the end cells 2 b themselves.

If the end cells 2 b generate electrical power with a current density of0.2 A/cm² at −20 degrees Centigrade, the heat generation amount due tothe power generation reaction will be about 0.2 W/cm². Herein, currentdensity means the current per square centimeter of the MEA 3.

In this condition, it takes the temperature of the end cells 2 b about 2minutes to rise to 0 degrees Centigrade. Due to the power generationreaction in this interval, about 2 milligrams per square centimeter(mg/cm²) of moisture is produced.

Hence, regarding the end cells 2 b, the MEA 3 is designed so that theMEA 3 can absorb all the moisture produced from when the temperature is−20 degrees Centigrade to when it reaches 0 degrees Centigrade. In thisway, flooding in the end cells 2 b below freezing point can beprevented. Some of the moisture produced by the cathode 26 b evaporatesfrom the solid electrolyte membrane 20 into the anode 26 a by a backdiffusion phenomenon, and this evaporation amount is not taken intoconsideration. However, if the amount of evaporation into the anode 6 acan be determined, it can be taken into account in the calculation ofthe moisture absorption capacity of the end cells 2 b. The abovecalculation is an example, and there will be a different value dependingon the composition of the MEA 3, or assumptions concerning the heatbalance.

Next, it will be described how to make the moisture absorption capacityof the MEA 3 of the end cells 2 b larger than the MEA 3 of the centercells 2 a.

For all the fuel cells of the fuel cell stack 1, it is also possible tovary the moisture absorption capacity from the center gradually towardboth ends.

However, this setup complicates the design, assembly and manufacture ofthe fuel cell stack 1. Moreover, increasing the moisture absorptioncapacity of all cells may reduce the power generation efficiency of thefuel cell stack 1 in the steady state.

In this embodiment, the center cells 2 a have the usual moistureabsorption capacity, and the end cells 2 b have a larger moistureabsorption capacity.

As mentioned above, there are five of the end cells 2 b at each end,although this number can be varied. The moisture absorption capacity isequal to the weight change of the MEA 3 when the MEA 3 is dried from thestate where the MEA 3 is humidified using humidifying gas having ahumidity of 100%.

(1) Thickness of Solid Electrolyte Membrane 20

The solid electrolyte membrane 20 using NAFION 111 perfluorosulfonicacid polymer having a film thickness of 30 μm can absorb 0.35 mg/cm² ofmoisture. The solid electrolyte membrane 20 using NAFION 117 of filmthickness 175 μm can absorb 2.1 mg/cm² of moisture. Hence, firstly forthe end cells 2 b, the film thickness of the solid electrolyte membrane20 is made larger than that for the center cells 2 a. For example, themoisture absorption capacity of the cells 2 b can be made larger thanthe moisture absorption capacity of the cells 2 a by using NAFION 111for the cells 2 a, and using NAFION 117 for the cells 2 b.

According to “Characterization of Polymer Electrolytes for Fuel Cells”by T. Zawodzinski et al. (J. Electrochem. Soc. 140 (1993) pp1981), whena solid electrolyte membrane is dried to some extent, the moisturecontent is 2, and when it has absorbed moisture to the maximum extent,the moisture content is about 6.

Apart from the moisture absorption capacity, as for the thickness of thesolid electrolyte membrane 20, it preferably has a thickness of 5 μm ormore from the viewpoint of making it impermeable to gas.

(2) Layer Thickness and Specific Surface Area of Electrodes 26 a, 26 b

If Vulcan carbon black having a specific surface area of 100 m²/g isused for the carbon material of the catalyst layer 21 or gas diffusionlayer 22, 0.1 mg/cm² of moisture can be adsorbed by a thickness of 5 μm.If the thickness of the electrode is 50 μm, 1 mg/cm² of moisture can beadsorbed.

If Ketjen black of specific surface area 1000 m²/g is used as the carbonmaterial of the catalyst layer 21 or gas diffusion layer 22, 2 mg/cm² ofmoisture can be adsorbed by a thickness of 10 μm.

Hence, by making the layer thickness of the electrodes of the cell 2 blarger than those of the cell 2 a, the moisture absorption capacity ofthe cell 2 b can be made larger than the moisture absorption capacity ofthe cell 2 a. Alternatively, the same result can be obtained by makingthe specific surface area of the electrodes of the cell 2 b larger thanthat of the cell 2 a.

The layer thickness of an electrode is proportional to the carbon amountused, and in order to support a sufficient amount of the platinum (Pt)catalyst, it is preferably a thickness of 5 μm or more.

(3) Ion Exchange Group Equivalent Weight EW

By using DOWEX perfluorosulfonic acid polymer having an EW of 800instead of the NAFION® film having an ion exchange group equivalentweight EW for the solid electrolyte membrane 20, approximately 1.5 timesthe amount of moisture can be absorbed. If the ion exchange groupequivalent weight EW is small, the number of moles of ion exchangegroups increases, so the moisture absorption capacity is enhanced. Ifthe thickness of the solid electrolyte membrane 20 is 30 μm, the DOWEX®film can absorb 0.53 mg/cm² of moisture. This is approximately 0.18mg/cm² more than a film of NAFION 112 of identical thickness. However,if the ion exchange group equivalent weight EW becomes small, the filmstrength decreases. In order to maintain a desirable film strength, theiron exchange group equivalent weight EW is preferably 200 or more.

(4) Admixture of Absorbing Material

The moisture absorbing capacity of the MEA 3 can be increased by mixingan absorbing material in the solid electrolyte membrane 20, catalystlayer 21 or the carbon layer which does not support platinum (Pt)provided adjacent to the catalyst layer 21. One example of such anabsorbing material is silica gel. If the silica gel content is 10%,approximately 2 mg/cm² of moisture can be absorbed by a film having athickness of 175 micrometers. The effect of mixing the absorbingmaterial can be calculated from isothermal adsorption curves.

U.S. Pat. No. 5,523,181 discloses a relation between the silica gelcontent and the moisture absorbing capacity of a solid electrolytemembrane.

The absorbing material may be a hygroscopic inorganic porous particlemoisture-absorbing resin selected from a group comprising silica gel,synthetic zeolite, alumina gel, titania gel, zirconia gel, yttria gel,tin oxide and tungsten oxide. Alternatively, any of a crosslinkedpolyacrylate, starch-acrylate graft copolymer cross-linked material,Poval polymer resin, polyacrylonitrile polymer resin or carboxymethylcellulose polymer resin is used. By using any of these, the moistureabsorbing capacity can be increased without causing a deterioration ofthe MEA 3.

Herein, the moisture-absorbing material is mixed with the solid polymerelectrolyte 20 only in the end cells 2 b. If on the other hand themoisture-absorbing material is mixed with the solid electrolyte membrane20 of both the end cells 2 b and center cells 2 a, themoisture-absorbing material admixture rate of the cells 2 b can bearranged to be higher than the moisture-absorbing material admixturerate of the cells 2 a. The moisture-absorbing material is preferablyarranged to lie within a range of 0.01 wt%-30 wt% relative to thepolymer electrolyte which is the main ingredient of the solidelectrolyte membrane 20. If it is less than 0.01%, the mixing of theabsorbing material cannot be expected to have any effect, and if itexceeds 30%, the proton electrical conduction rate of the solidelectrolyte membrane 20 considerably decreases.

(5) Polymer Liquid Capacity During Manufacture of the MEA 3

When the MEA 3 contains approximately 0.5 mg/cm² of NAFION®, it canabsorb 0.02 mg/cm² of moisture. This can be implemented by coating acatalyst containing NAFION® to the catalyst layer 21. If a NAFION®solution is also coated on the gas diffusion layer 22, the MEA 3 canhold approximately 50 gm/cm² of NAFION®, and can therefore absorb 2mg/cm² of moisture. Hence, the polymer solution having a perfluorocarbonsulfonic acid as its main starting material has moisture-absorbingability.

By arranging that the end cells 2 b contain more of the polymer solutionthan the center cells 2 a, the moisture absorption capacity of the endcells 2 b can be made larger than the moisture absorption capacity ofthe center cells 2 a.

The polymer weight required to form a three-phase interface in thecatalyst layer 21 is 0.1 mg/cm², and the usage amount of the polymersolution is preferably determined to satisfy this requirement.

By using one or plural methods selected from the aforesaid (1)-(5), themoisture absorption capacity of the MEA 3 of the end cells 2 b can bemade larger than the moisture absorption capacity of the MEA 3 of thecenter cells 2 a. Among the aforesaid methods (1)–(5), the method usedto increase the moisture absorption capacity of the gas diffusion layer22 or catalyst layer 21 may be applied to the gas diffusion layer 22 andcatalyst layer 21 of both the anode 26 a and cathode 26 b, or may beapplied to the gas diffusion layer 22 or catalyst layer 21 of only thecathode 26 b. Since it is the cathode 26 b which produces moisture bythe electrochemical reaction, there is a tendency for flooding to occurat the cathode 26 b more than at the anode 26 a.

The setting of the moisture-absorbing amount is not limited to these twotypes, and more settings are possible. As described above, for all thefuel cells in the fuel cell stack 1, the moisture absorption capacitymay also be gradually varied from the center to the two ends.

According to this embodiment, by applying one or more of the aforesaidmethods, the moisture absorption capacity of the end cell 2 b is set tobe larger than the moisture absorption capacity of the center cell 2 a.

Next, the control of the humidifiers 6, 7 which affect the moisturecontent of the fuel cell stack 1, and the constant current controlcircuit 8 which adjusts the output current of the fuel cell stack 1 andtherefore varies the heat generation amount in the fuel cells 1, will bedescribed.

To control these units, the fuel cell system comprises a controller 11.The controller 11 respectively controls the humidification state of thehydrogen and air supplied to the fuel cell stack 1 by signals output tothe humidifiers 6, 7. The controller 11 also controls the output currentof the fuel cell stack 1 by outputting a signal to the constant currentcontrol circuit 8.

The controller 11 comprises a microcomputer comprising a centralprocessing unit (CPU), read-only memory (ROM), random access memory(RAM) and input/output interface (I/O interface). The controller mayalso comprise plural microcomputers.

In order for the controller 11 to control the humidifiers 6, 7 and theoutput current of the fuel cell stack 1, detection data are input assignals to the controller 11 respectively from a temperature sensor 17which detects the temperature of the center cells 2 a, a current sensor12 which detects the output current of the fuel cell stack 1, and avoltage sensor 13 which detects the output voltage of the fuel cellstack 1.

Next, the routine for stopping the fuel cell system executed by thecontroller 11 will be described. The controller 11 executes this routinewhen an operation stop command is input to the input interface.

Referring to FIG. 3, first in a step S1, the controller 11 stops powergeneration of the fuel cell stack 1 by issuing a command to the constantcurrent control circuit 8 which sets the output current to zero.

In a following step S2, the controller 11 estimates the moisture contentof the fuel cells 2 a, 2 b. This estimation is performed based on thedetection values of the current sensor 12 and voltage sensor 13immediately prior to stopping power generation by the fuel cell stack 1.Specifically, the larger is the output current of the fuel cell stack 1,the more vigorous is the electrochemical reaction of the fuel cells 2 a,2 b and the larger is the moisture amount produced, so it is consideredthat the moisture content of the solid electrolyte membrane 20 of thefuel cells 2 a, 2 b is then larger. In other words, it is consideredthat the larger is the power output of the fuel cell stack 1, the largeris the moisture content of the solid electrolyte membrane 20 of the fuelcells 2 a, 2 b.

A humidity sensor may of course also be installed in the fuel cell stack1, and the moisture content of the solid electrolyte membrane 20 of thefuel cells 2 a, 2 b may be determined based on the humidity detected bythe humidity sensor. A 1 kHz high frequency impedance meter whichmeasures the membrane resistance of the MEA 3, and a thermocouple whichmeasures temperature, may also be installed in one of the fuel cells 2a, 2 b, and the moisture content of the fuel cell stack 1 may beestimated from the membrane resistance and temperature.

In a next step S3, the controller 11 performs drying of the fuel cells 2a, 2 b. Specifically, humidification by the humidifiers 6, 7 is firststopped, and hydrogen from the hydrogen supply passage 14 and air fromthe air supply passage 15 are supplied to the fuel cell stack 1 to purgeinternal moisture. Although hydrogen and air are supplied to the fuelcells 2 a, 2 b, the output current is suppressed to zero by the constantcurrent control circuit 8, so the fuel cells 2 a, 2 b do not perform anelectrochemical reaction. The hydrogen and air supply time is arrangedto be longer, the larger is the moisture content estimated in the stepS2. Instead of completely stopping the humidifiers 6, 7, the moisture inthe fuel cells 2 a, 2 b may be purged using low humidity hydrogen andair as a low humidification state.

Part of the moisture impregnated in the solid electrolyte membrane 20 ispurged by the dried or low humidity hydrogen supplied to the anode 26 a,flows into the combustor 19 together with the hydrogen, and is processedby the combustor 19. Alternatively, part of the moisture impregnated inthe solid electrolyte membrane 20 is discharged to the atmospheretogether with air purged by the dried or low humidity air supplied tothe cathode 26 b.

The purge duration time is set based on the following condition.Specifically, the purge duration time must be such that the moisturecontent allows power generation even for startup below freezing point,and such that the moisture amount produced until the temperature of thefuel cells 2 a, 2 b rises above freezing point due to theelectrochemical reaction on startup, remains within a range in whichflooding does not occur.

When the purge continuation time reaches the time set in this way, in astep S4, the controller 11 stops the supply of hydrogen and air to thefuel cell stack 1. Operation of other auxiliary devices of the fuel cellstack 1 is also stopped.

This routine is executed when operation of the fuel cell system stops,but may also be executed according to the temperature drop after thesystem stops operating. In this case, even after the fuel cell systemstops operating, the controller 11 is kept in an active state. Thecontroller 11 monitors the temperature sensor 17 after operation stops,and when the temperature of the fuel cells 2 a, 2 b approaches freezingpoint, executes the processing of the step S3.

In this case, provided that the fuel cells 2 a, 2 b do not fall to lowtemperature, purge is not performed. The purge condition may also be setbased on the atmospheric temperature instead of the temperature of thefuel cells 2 a, 2 b.

If the moisture in the fuel cells 2 a, 2 b freezes, the moisture in thefuel cells 2 a, 2 b can no longer be purged by hydrogen or air. In anycase, therefore, the routine must be executed before the temperaturefalls below freezing point.

In this fuel cell stack 1, the moisture absorption capacity of the endcells 2 b was set to be larger than that of the center cells 2 a, soeven if the whole fuel cell stack 1 was uniformly dried by the routineof FIG. 3, the end cells 2 b may absorb more moisture than the centercells 2 a on the next startup.

Instead of leaving the fuel cells 2 a, 2 b after they are completelydried, it is preferred to place the fuel cells 2 a, 2 b in a lowhumidity state in preparation for the next power generation. In thiscase, hydrogen and air which have been humidified to a low humiditystate are supplied to the fuel cell stack 1 after purge.

Next, the routine for starting up of the fuel cell stack 1 executed bythe controller 11 will be described referring to FIG. 4. The controller11 executes this routine when an operation command signal of the fuelcell system is input to the input interface.

First, in a step S11, the controller 11 reads the temperature of thefuel cell stack 1 detected by the temperature sensor 17.

In a following step S12, the controller 11 determines whether or not thetemperature of the fuel cell stack 1 is higher than freezing pointtemperature, i.e., zero degrees Centigrade.

If the temperature of the fuel cell stack 1 is higher than freezingpoint temperature, the controller 11 terminates the routine withoutperforming any further steps. In this case, the fuel cell systemperforms ordinary operation.

If the temperature of the fuel cell stack 1 is not higher than freezingpoint temperature, the controller 11, in a step S13, performs lowtemperature operation. Low temperature operation maintains the outputcurrent of the fuel cell stack 1 at a constant current via the constantcurrent control circuit 8 regardless of the power required from the fuelcell stack 1, and continues operation of the fuel cell system aimed atthawing the frozen moisture in the fuel cells 2 a, 2 b for a fixed time.

After operation of the fuel cell system has continued for a fixed time,the controller 11 repeats the processing of the steps 11-13 until thetemperature of the fuel cell stack 1 rises above freezing pointtemperature.

If the fuel cell system is used as a motive power source to run avehicle, the power obtained from power generation to thaw the fuel cellstack 1 can be used to run the vehicle. Further, the extra power can beused for heating auxiliary equipment, stack itself, or charging thebattery.

Next, the effect of this fuel cell system will be described referring toFIGS. 5A-5F.

Among FIGS. 5A-5F, FIGS. 5D-5F show the startup state of this fuel cellsystem wherein the moisture absorption capacity of the end cells 2 b isarranged to be larger than that of the center cells 2 a. FIGS. 5A-5Cshow the startup state in the prior art wherein the moisture absorptioncapacity of all cells is uniform. The high voltage immediately prior tooperation startup in FIGS. 5B, 5C shows the open-circuit voltage.

At −20 degrees Centigrade, when the fuel cell system according to theprior art technique is started up, and some time has elapsed after thefuel cell starts generating power on startup, due to the moisture whichhas thawed due to the heat generated by the electrochemical reaction orthe moisture produced by the electrochemical reaction, flooding mayoccur in the cathode of the fuel cell before the temperature of the fuelcell stack 1 shown in FIG. 5C reaches 0 degrees Centigrade. FIG. 5Ashows the sharp fall in fuel cell output current and FIG. 5B shows thesharp fall in fuel cell output voltage when flooding occurs. If thissituation develops, the fuel cell stops generating power while it isstill in the frozen state. Flooding tends to occur more easily in thefuel cells situated at the ends of the fuel cell stack, as mentionedabove.

On the other hand, in the fuel cell system according to this invention,the moisture absorption capacity of the end cell 2 b is arranged to belarger than that of the center cells 2 a, so the probability that theend cell 2 b will flood until the temperature of the fuel cell stack 1rises above freezing point, is less than that in the prior arttechnique. Therefore, as shown in FIGS. 5D, 5E, the output current andoutput voltage of the fuel cells 2 a, 2 b do not fall. Further, as thefuel cells 2 a, 2 b actively generate power when hydrogen and air aresupplied, the temperature of the fuel cell stack 1 can be raised toabove freezing point in a short time, as shown in FIG. 5F.

Next, a second embodiment of this invention will be described referringto FIGS. 6 and 7.

According to this embodiment, the moisture absorption capacity of theMEA 3 used by the center cell 2 a and end cell 2 b are made identical.

On the other hand, the drying extents of the MEA 3 of the center cell 2a and end cell 2 b before the fuel cell stack 1 has fallen belowfreezing point, are arranged to be different, and as a result, adifference arises in the moisture absorption performance when the fuelcell stack 1 starts up.

In a fuel cell system according to this embodiment, two hydrogen supplypassages 14 a, 14 b, and two air supply passages 15 a, 15 b, areprovided.

The hydrogen supply passage 14 a supplies hydrogen to the anode of thecenter cell 2 a via a center manifold 16 a. The hydrogen supply passage14 b supplies hydrogen to the anode of the end cell 2 b via an endmanifold 16 b.

The air supply passage 15 a supplies air to the cathode of the centercell 2 a via the center manifold 16 a The air supply passage 15 bsupplies air to the cathode of the end cell 2 b via the end manifold 16b.

The center manifold 16 a houses a hydrogen branch tube leading to theanode of each of the cells 2 a, and an air branch tube leading to thecathode of each of the cells 2 a. The end manifold 16 b houses ahydrogen branch tube leading to the anode of each of the cells 2 b, andan air branch tube leading to the cathode of each of the cells 2 b.

Pressure adjusting devices 14 c, 14 d for adjusting pressure arerespectively installed in the hydrogen supply passages 14 a, 14 b.Pressure adjusting devices 15 c, 15 d for adjusting pressure arerespectively installed in the hydrogen supply passages 15 a, 15 b.

The remaining features of the hardware construction of the fuel cellsystem are identical to the fuel cell system according to the firstembodiment.

The controller 11, when the fuel cell system stops operating, executesthe routine of FIG. 7 instead of the routine of FIG. 3.

In this routine, a step S30 is provided instead of the step S3 of theroutine of FIG. 3.

In the step S30, the set pressure of the pressure adjusting devices 14d, 15 d is set larger than the set pressure of the pressure adjustingdevices 14 c, 15 c so that when the fuel cell system stops operating,the moisture content of the MEA 3 of the cells 2 b is lower than themoisture content of the MEA 3 of the cells 2 a.

Specifically, the pressure of hydrogen supplied from the hydrogen supplypassage 14 b to the cells 2 b is arranged to be higher than the pressureof hydrogen supplied from the hydrogen supply passage 14 a to the cells2 a, and the pressure of air supplied from the air supply passage 15 bto the cells 2 b is arranged to be higher than the pressure of airsupplied from the air supply passage 15 a to the cells 2 a.

Instead of the pressure adjusting devices 14 c, 15 c, 14 d, 15 d, it ispossible to use adjusting devices for flow rate, humidity ortemperature. It is also possible to set different supply times forhydrogen and air to the cells 2 a and to the cells 2 b.

Concerning the hydrogen and air supplied to the cells 2 b, the moisturecontent of the MEA 3 of the cells 2 b can be arranged lower than themoisture content of the MEA 3 of the cells 2 a when the fuel cell systemstops operating by increasing the flowrate, decreasing thehumidification, increasing the temperature or increasing the supplytime.

The processing of the other steps S1, S2 and S4 is identical to theroutine of FIG. 3 of the first embodiment.

When the fuel cell system starts up, the controller 11 also executes theroutine of FIG. 4 in an identical manner to the first embodiment.

The moisture absorption capacity of the MEA 3 depends on the moisturecontent when the MEA 3 starts absorbing moisture, and more moisture canbe absorbed the lower is the moisture content when moisture absorptionstarts, i.e., the drier is the MEA 3. In this embodiment, the moisturecontent of the MEA 3 of the cells 2 b when the fuel cell system stopsoperating is arranged to be lower than the moisture content of the MEA 3of the cells 2 a. As a result of this measure, when the fuel cell systemstarts operating again, the MEA 3 of the cells 2 b can absorb moremoisture. Therefore, as in the first embodiment, flooding in the fuelcells 2 a, 2 b when the fuel cell system starts up below freezing pointcan be prevented. Consequently, startup of the fuel cell system is notimpeded by freezing of excess moisture due to flooding, and the start-upperformance of the fuel cell system can be enhanced.

According to this embodiment, the same desirable effect as that of thefirst embodiment regarding prevention of flooding on startup can beobtained simply by purge control without changing the specifications ofthe center cells 2 a and end cells 2 b.

In this embodiment, in addition to the difference of moisture contentwhen operation stops, the difference in moisture absorption capacity ofthe first embodiment can also be incorporated.

In this embodiment, as in the first embodiment, a construction may beadopted wherein the MEA 3 used in the center cells 2 a and the MEA 3used in the end cells 2 b have different moisture absorption functions.

In this embodiment, the four pressure adjusting devices 14 c, 14 d, 15c, 15 d are provided, but it is not absolutely necessary to provide allthese adjusting devices. If at least the pressure adjusting device 14 drelating to supply of air to the end cells 2 b is provided, the moisturecontent of the MEA 3 of the cells 2 b can be arranged to be lower thanthe moisture content of the MEA 3 of the cells 2 a. The pressureadjusting devices 14 c, 14 d correspond to the purge device in theclaims.

In the aforesaid embodiments, moisture was purged using hydrogen and airin order to dry the MEA 3. However, instead of gases used in theelectrochemical reaction such as hydrogen and air, moisture can bepurged also by supplying a special purge gas such as for examplenitrogen gas or combustion gas from the combustor 19.

Further, in the fuel cells 2 a, 2 b, moisture is produced by theelectrochemical reaction in the cathode 26 a. Therefore, purging may beperformed only for the cathode 26 b without purging the anode 26 a.

Next, referring to FIGS. 8-10, a third embodiment of this invention willbe described.

In this embodiment, NAFION 111 having a film thickness of 30 μm is usedfor the solid electrolyte membrane 20 of the center cells 2 a, andNAFION 112 having a film thickness of 50 μm is used for the end cells 2b. Carbon paper of thickness 200 μm is used for the gas diffusion layer22 of the center cells 2 a, and carbon paper of thickness 300 μm is usedto the gas diffusion layer 22 of the end cells. Moisture repellingtreatment is given to the carbon paper. To enhance the moisture removalproperties of the end cells 2 b, the amount of moisture repellingmaterial in the end cells 2 b is preferably arranged to be larger thanthat of the center cells 2 a. Further, the diameter of the pores in thecarbon paper used in the end cells 2 b is preferably arranged to belarger than the diameter of the pores in the carbon paper used in thecenter cells 2 a. For this purpose, instead of the carbon paper in theend cells 2 b, carbon cloth could be used.

As a method of increasing the moisture absorption capacity, the contactangle between the gas diffusion layer 22 and moisture is preferablyincreased. Specifically, this is done by forming fine irregularities inthe surface of the carbon paper forming the gas diffusion layer 22. Morespecifically, when the carbon paper is given a moisture-repellanttreatment, the surface of the carbon paper is roughened by mixing afluorine compound such as polytetrafluoroethylene (TEFLON®) and siliconoxide (SiO₂) particles.

As another method of increasing the contact angle between the gasdiffusion layer 22 and moisture, the hydrophilicproperties/moisture-repelling properties of the diffusion layer 22 mayalso be varied. In the case of TEFLON, the contact angle is 108 degrees,but poly perfluorooctylethylacrylate which has been given a higherfluorine concentration has a contact angle of 120 degrees. Whereas thefluorine in TEFLON is bonded in the form of —CF₂—CF₂—,perfluorooctylethyl acrylate has the side chains —CF₂—CF(CF₃)—, so thefluorine concentration is higher.

Referring to FIG. 8, as in the second embodiment, the fuel cell systemaccording to this embodiment supplies hydrogen and air to the fuel cellstack 1 having the aforesaid construction respectively using the twohydrogen supply passages 14 a, 14 b, and the two air supply passages 15a, 15 b.

Hydrogen is supplied to the hydrogen supply passages 14 b, 14 b from ahydrogen tank.

A valve 18 a and humidifier 6 a are installed in the hydrogen supplypassage 14 a, and a valve 18 b and humidifier 6 b are installed in thehydrogen supply passage 14 b, respectively, so that the hydrogen supplyflowrate and degree of humidification of the hydrogen supply passages 14a, 14 b can be separately adjusted.

A valve 27 a and humidifier 7 a are installed in the air supply passage15 a, and a valve 27 b and humidifier 7 b are installed in the airsupply passage 15 b, respectively, so that the air supply flowrate anddegree of humidification of the air supply passages 15 a, 15 b can beseparately adjusted.

In this embodiment, instead of the temperature sensor 17 used in thesecond embodiment, a thermocouple 28 is installed in one of the endcells 2 b. The thermocouple 28 detects the temperature of the end cell 2b, and inputs a corresponding signal to the controller 11.

The remaining features of the construction of the fuel cell system areidentical to those of the second embodiment.

In this fuel cell system, the controller 11 performs the routine of FIG.7 when the system stops operation and performs the routine of FIG. 4when it starts operation as in the case of the second embodiment.

Next, referring to FIG. 9, the power supply start control of this fuelcell system normal will be described. The controller 11 executes theroutine shown in FIG. 9 after the completion of the startup routine ofFIG. 4.

First, in a step S21, the controller 11 operates the valves 18 a, 18 band humidifiers 6 a, 6 b to supply high humidity hydrogen to the endcells 2 b, and supply low humidity hydrogen to the center cells 2 a.Likewise, the controller 11 operates the valves 27 a, 27 b andhumidifiers 7 a, 7 b to supply high humidity air to the end cells 2 b,and supply low humidity air to the center cells 2 a.

For example, if the temperature of the fuel cell system on startup is 20degrees Centigrade there is no risk that the fuel cells 2 a, 2 b willfreeze even if the supplied hydrogen and air are humidified. Therefore,under these conditions, hydrogen and air of 100% humidity can besupplied to the end cell 2 b, while hydrogen and air having a humidityof 80% can be supplied to the center cell 2 b.

In a next step S22, the output voltage of the fuel cell stack 1 detectedby the voltage sensor 13 is compared with a predetermined voltage.Herein, the fuel cell stack 1 is not supplying power to the electricalload 10, so the detection value of the voltage sensor 13 corresponds tothe open-circuit voltage of the fuel cell stack 1. The predeterminedvoltage is the rated output voltage of the fuel cell stack 1.

When the output voltage of the fuel cell stack 1 is less than thepredetermined voltage, the controller 11 repeats the processing from thestep S21.

When the output voltage of the fuel cell stack 1 reaches thepredetermined voltage, the controller 11, in a step S23, controls theconstant current control circuit 8 to start supplying power to the load10.

In a next step S24, the controller 11 decreases the humidity of thehydrogen and air supplied to the fuel cell stack 1 so that flooding inthe fuel cell stack 1 does not occur. After the processing of the stepS24, the controller 11 terminates the routine, and shifts to control forsteady-state operation. When the temperature in steady-state operationof the center cells 2 a of the fuel cell stack 1 is 70 degreesCentigrade, as heat escapes from the end plate 5, the temperature of theend cells 2 b is a value 2-3 degrees Centigrade lower than that of thecenter cells 2 a. Consequently, during steady-state operation, if thehumidity of the hydrogen and air supplied to the center cells 2 a andend cells 2 b is set equal to the humidity set in the step S21, there isa possibility that flooding will occur. Also, as described above, thethickness of the solid electrolyte membrane 20 is larger for the endcells 2 b than for the center cells 2 a, so the moisture displacementamount from the cathode 26 b to the anode 26 a via the solid electrolytemembrane 20 is small. In view of this characteristic, duringsteady-state operation, hydrogen and air humidified to 50% humidity aresupplied to the center cells 2 a, and hydrogen and air humidified to 20%humidity are supplied to the end cells 2 b.

There is a possibility that flooding will occur even during steady-stateoperation of the fuel cell system. Hence, even during steady-stateoperation, the controller 11 performs drying of the fuel cells 2 a, 2 bat an interval of from several minutes to several tens of minutes byexecuting the routine shown in FIG. 10. The drying operation isperformed separately for the center cells 2 a and end cells 2 b.Specifically, in the end cells 2 b, the humidity of hydrogen and air islowered, the supply flowrate is increased or the drying operation timeis increased compared to the center cell 2 a. In this embodiment, thisprocedure is performed for both the hydrogen supplied to the anode 26 aand the air supplied to the cathode 26 b, but as flooding occurs mainlyin the cathode 26 b, the drying procedure may be applied only to air.

The routine of FIG. 10 is executed at an interval of one hundredmilliseconds during steady-state operation of the fuel cell stack 1.

Referring to FIG. 10, first in a step S31, the controller 11 determineswhether or not a predetermined time has elapsed from when the fuel cellstack 1 started generating power. If the determined time has notelapsed, the routine is immediately terminated without proceeding tofurther steps.

If the predetermined time after power generation start has elapsed, thecontroller 11, in a step S32, determines whether or not a drying flag isunity. The initial value of the drying flag is zero.

When the drying flag is not unity, the controller 11, in a step S36,determines whether or not the state where the drying flag is zero hasreached a predetermined time.

When the state where the drying flag is zero has reached a predeterminedtime, the controller 11, in a step S37, sets the drying flag to unity,and terminates the routine. When the state where the drying flag is zerohas not reached the predetermined time, the controller 11 terminates theroutine without performing other processing.

On the other hand, in the step S32, when the drying flag is unity, thecontroller 11, in a step S33, performs the drying procedure separatelyfor the end cells 2 b and the center cells 2 a. Specifically, thehumidification of the hydrogen and air supplied to the end cells 2 b andcenter cells 2 a is decreased by operating the humidifiers 6 a, 6 b andhumidifiers 7 a, 7 b. Also, the hydrogen and air supply flowratessupplied to the end cells 2 b and center cells 2 a are varied byoperating the valves 18 a, 18 b and the valves 27 a, 27 b. Due to thisprocessing, both the end cells 2 b and center cells 2 a are dried, anddue to the control difference in the procedure, the dryness of the endcells 2 b is enhanced compared to that of the center cells 2 a.

In a next step S34, it is determined whether or not the drying procedureis complete. This determination is made based on the drying continuationtime. Alternatively, by using the method described for the routine ofFIG. 3, the humidity may actually be detected, and it may be determinedwhether the drying procedure is complete by comparing it with apredetermined target humidity.

In the step S34, if it is determined that the drying procedure iscomplete, the controller 11, in a step S35, resets the drying flag tozero, and terminates the routine. If it is determined that the dryingprocedure is not complete, the controller 11 terminates the routinewithout changing the setting of the drying flag.

According to this routine, the drying procedure is performed for thefirst time when the predetermined time has elapsed after the fuel cellstack 1 starts operating, and when the drying procedure is complete, thedrying procedure is again performed when the predetermined time haselapsed. In this way, by regularly performing the drying procedureduring steady-state operation, flooding during steady-state operationcan be prevented.

In this embodiment, the drying procedure is performed at a predeterminedinterval, but it is also possible to perform the drying procedure whenthe fuel cell stack 1 is not generating power as in the case when avehicle is decelerating.

Next, a fourth embodiment of this invention will be described.

This embodiment relates to the construction of the fuel cell stack 1.

In this embodiment, the cross-sections of the passages 24, 25 formed inthe separators 23 of the end cells 2 b of the fuel cell stack 1, are setto be larger than the cross-sections of the passages 24, 25 formed inthe separators 23 of the center cells 2 a. Specifically, thecross-sections of the passages 24, 25 of the end cells 2 b are set tohave a width of 1 millimeter (mm) and a depth of 1 mm, whereas thecross-sections of the passages 24, 25 of the center cells 2 a are set tohave a width of 0.8 mm and a depth of 0.8 mm.

The construction of the fuel cell system other than the fuel cell stack1 is identical to the second embodiment shown in FIG. 6.

For example, when the fuel cell system is started in a low-temperatureenvironment of −20 degrees centigrade, if there is ice in the passages24, 25 of the fuel cells 2 a, 2 b in the fuel cell stack 1, the cellsmay not be able to produce an open-circuit voltage. Hence, by making thepassages 24, 25 of the end cells 2 b which tend to fall to lowtemperature, larger than those of the center cell 2 a, the probabilityof a state in which power cannot be generated is reduced. This isconsidered to be due to the fact that the passage is less easily blockedby ice, the larger is the cross-section of the passage. It may be notedthat if flowrate control of the hydrogen supply passages 14 a, 14 b andair supply passages 15 a, 15 b is also performed so that the hydrogensupply flowrate to the end cells 2 b is larger than the hydrogen supplyflowrate to the center cells 2 a, and so that the air supply flowrate tothe end cells 2 b is larger than the air supply flowrate to the centercells 2 a, an even more marked effect can be expected.

INDUSTRIAL FIELD OF APPLICATION

As described above, according to this invention, flooding of the endcells of a fuel cell stack at low temperature can be prevented.Therefore, by applying this invention to a fuel cell system for avehicle which often starts up under low temperature, it has a desirableeffect in enhancing the low-temperature start-up performance of thevehicle.

The contents of Tokugan 2003-136791, with a filing date of May 15, 2003in Japan, are hereby incorporated by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art,within the scope of the claims.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A fuel cell system comprising: a fuel cell stack comprising aplurality of fuel cells stacked in series; wherein the fuel cellscomprise a first fuel cell disposed in a center position of the fuelcell stack with respect to a stacking direction of the fuel cells, and asecond fuel cell disposed in a position other than the center positionwith respect to the stacking direction of the fuel cells, the secondfuel cell being arranged to have a larger moisture absorption capacitythan the first fuel cell.
 2. The fuel cell system as defined in claim 1,wherein each of the fuel cells comprises an anode to which hydrogen issupplied, a cathode to which air containing oxygen is supplied, and aelectrolyte membrane formed of a moisture-absorbing material whichconducts hydrogen ions from the anode to the cathode, and theelectrolyte membrane of the second cell is formed to have a largermoisture absorption capacity than the electrolyte membrane of the firstcell.
 3. The fuel cell system as defined in claim 1, wherein each of thefuel cells comprises an anode to which hydrogen is supplied, a cathodeto which air containing oxygen is supplied, and a electrolyte membraneformed of a moisture-absorbing material which conducts hydrogen ionsfrom the anode to the cathode, and the cathode of the second cell isformed to have a larger moisture absorption capacity than the cathode ofthe first cell.
 4. The fuel cell system as defined in claim 1, whereinthe fuel cell system further comprises a humidifier which humidifies airsupplied to the fuel cells and a programmable controller programmed tocontrol the humidifier to suppress the humidification of the airsupplied to the fuel cells before the fuel cell system stops operating.5. The fuel cell system as defined in claim 4, wherein the fuel cellsystem further comprises a sensor which detects a parameter related tothe moisture content of the fuel cells, and the controller is furtherprogrammed to determine the moisture content of the fuel cells from theparameter, and to make the humidification suppression time longer, thelarger is the moisture content of the fuel cells.
 6. The fuel cellsystem as defined in claim 4, wherein the fuel cell system furthercomprises a humidifier which humidifies hydrogen supplied to the fuelcells, and the controller is further programmed to control thehumidifier to suppress the humidification of the hydrogen supplied tothe fuel cells before the fuel cell system stops operating.
 7. The fuelcell system as defined in claim 1, wherein the fuel cell system furthercomprises a humidifier which humidifies air supplied to the fuel cells,a sensor which detects a temperature of the fuel cell stack and aprogrammable controller which controls the humidifier, wherein thecontroller is programmed to supply air to the fuel cells whilesuppressing humidification of air by the humidifier, when thetemperature of the fuel cell stack reaches a predetermined lowtemperature region after the fuel cell system stops operating.
 8. Thefuel cell system as defined in claim 1, wherein the fuel cell systemfurther comprises an electrical circuit which adjusts an output currentof the fuel cell stack, a sensor which detects a temperature of the fuelcell stack and a programmable controller programmed to control theelectrical circuit to maintain the output current of the fuel cell stackat a constant current when the temperature of the fuel cell stack is ina predetermined low temperature region when the fuel cell system startsup.
 9. The fuel cell system as defined in claim 1, wherein the fuel cellsystem further comprises a purge device which purges residual moisturein the second cell, a sensor which detects a temperature of the fuelcell stack and a programmable controller programmed to operate the purgedevice so that the moisture content of the second cell is less than themoisture content of the first cell when the temperature of the fuel cellstack is in a predetermined low temperature region.
 10. The fuel cellsystem as defined in claim 9, wherein the purge device is a device whichadjusts one of a pressure, a flowrate, a humidification degree, atemperature and a supply time of the air supply to the second cell. 11.The fuel cell system as defined in claim 1, wherein the fuel cell systemfurther comprises a first humidifier which humidifies air supplied tothe first cell, a second humidifier which humidifies air supplied to thesecond cell, a sensor which detects a temperature of the fuel cell stackand a programmable controller programmed to control the first humidifierand second humidifier so that the humidity of the air supplied to thesecond cell is higher than the humidity of the air supplied to the firstcell when the temperature of the fuel cell stack is equal to or higherthan a predetermined temperature when the fuel cell system starts up.12. The fuel cell system as defined in claim 1, wherein the fuel cellsystem further comprises a first humidifier which humidifies airsupplied to the first cell, a second humidifier which humidifies airsupplied to the second cell, a voltage sensor which detects an outputvoltage of the fuel cell stack and a programmable controller programmedto control the first humidifier and second humidifier so that thehumidity of the air supplied to the first cell is higher than thehumidity of the air supplied to the second cell when the output voltageof the fuel cell stack is equal to or greater than a predeterminedvoltage.
 13. The fuel cell system as defined in claim 1, wherein thefuel cell system further comprises a first humidifier which humidifiesair supplied to the first cell, a second humidifier which humidifies airsupplied to the second cell and a programmable controller programmed tocontrol the first humidifier and second humidifier to decrease thehumidity of the air supplied to both the first cell and second cell at apredetermined interval in a state where the humidity of the air suppliedto the second cell is lower than the humidity of the air supplied to thefirst cell.
 14. A fuel cell stack generating electric power throughelectrochemical reaction of hydrogen and oxygen, comprising: a pluralityof fuel cells stacked in series, each of the fuel cells comprising ananode to which hydrogen is supplied, a cathode to which air containingoxygen is supplied, and an electrolyte membrane which conducts hydrogenions from the anode to the cathode; wherein the fuel cells comprise afirst cell disposed in a center position of the fuel cell stack withrespect to a stacking direction of the fuel cells, and a second celldisposed in a position other than the center position, the second cellbeing arranged to have a larger moisture absorption capacity than thefirst cell.
 15. The fuel cell stack as defined in claim 14, wherein theelectrolyte membrane comprises a moisture-absorbing material, and theelectrolyte membrane of the second cell has a larger thickness in thestacking direction than the electrolyte membrane of the first cell. 16.The fuel cell stack as defined in claim 14, wherein the electrolytemembrane comprises a moisture-absorbing material, and the electrolytemembrane of the second cell has a larger ion exchange group equivalentweight than the electrolyte membrane of the first cell.
 17. The fuelcell stack as defined in claim 14, wherein the second cell comprises asubstrate material, and a moisture-absorbing material mixed with thesubstrate material.
 18. The fuel cell stack as defined in claim 17,wherein the moisture-absorbing material is a material selected from agroup comprising hygroscopic inorganic porous particlemoisture-absorbing resins comprising silica gel, synthetic zeolite,alumina gel, titania gel, zirconia gel, yttria gel, tin oxide andtungsten oxide.
 19. The fuel cell stack as defined in claim 17, whereinthe moisture-absorbing material is a material selected from a groupcomprising a crosslinked polyacrylate, starch-acrylate graft copolymercross-linked material, Poval polymer resin, polyacrylonitrile polymerresin and carboxymethylcellulose polymer resin.
 20. The fuel cell stackas defined in claim 17, wherein the substrate material is theelectrolyte membrane of the second cell, and the moisture-absorbingmaterial is mixed with the electrolyte membrane within a weight range of0.01% to 30% relative to a weight of the electrolyte membrane.
 21. Thefuel cell stack as defined in claim 17, wherein the cathode comprises acatalyst layer in contact with the electrolyte membrane and a cathodegas diffusion layer which diffuses oxygen in the air into the catalystlayer, the substrate material is the cathode gas diffusion layer of thesecond cell, and the moisture-absorbing material is mixed with thecathode gas diffusion layer within a weight range of 0.01% to 30%relative to a weight of the cathode gas diffusion layer of the secondcell.
 22. The fuel cell stack as defined in claim 14, wherein thecathode comprises a catalyst layer in contact with the electrolytemembrane and a cathode gas diffusion layer formed of amoisture-adsorbing material which diffuses oxygen in the air into thecatalyst layer, and the cathode gas diffusion layer of the second cellhas a larger thickness in the stacking direction than the cathode gasdiffusion layer of the first cell.
 23. The fuel cell stack as defined inclaim 14, wherein the cathode comprises a catalyst layer in contact withthe electrolyte membrane and a cathode gas diffusion layer formed of amoisture-adsorbing material which diffuses oxygen in the air into thecatalyst layer, and the cathode gas diffusion layer of the second cellhas a larger specific surface than the cathode gas diffusion layer ofthe first cell.
 24. The fuel cell stack as defined in claim 14, whereinthe anode, the cathode and the electrolyte membrane are formed of aone-piece membrane electrode assembly coated with a polymer solution,and the membrane electrode assembly of the second cell has a largerpolymer solution coating amount than the membrane electrode assembly ofthe first cell.
 25. The fuel cell stack as defined in claim 24, whereinthe polymer solution contains a perfluorocarbon sulfonic acid.
 26. Thefuel cell stack as defined in claim 14, wherein the cathode comprises acatalyst layer in contact with the electrolyte membrane and a cathodegas diffusion layer with numerous pores which diffuse oxygen in the airinto the catalyst layer, and the pores of the cathode gas diffusionlayer of the second cell have a larger diameter than the pores of thecathode gas diffusion layer of the first cell.
 27. The fuel cell stackas defined in claim 14, wherein the cathode comprises a catalyst layerin contact with the electrolyte membrane and a cathode gas diffusionlayer which diffuses oxygen in the air into the catalyst layer, and thecathode gas diffusion layer of the second cell is formed of a materialhaving a larger contact angle with moisture than the cathode gasdiffusion layer of the first cell.
 28. The fuel cell stack as defined inclaim 14, wherein the cathode comprises a catalyst layer in contact withthe electrolyte membrane and a cathode gas diffusion layer whichdiffuses oxygen in the air into the catalyst layer, the cathode gasdiffusion layer comprising carbon paper coated with moisture repellingmaterial, and the carbon paper of the cathode gas diffusion layer of thesecond cell has a larger amount of moisture repelling material than thecarbon paper of the cathode gas diffusion layer of the first cell. 29.The fuel cell stack as defined in claim 14, wherein the fuel cell stackcomprises plural end cells which have a progressively increasingmoisture-absorbing capacity with increasing distance from the centercell.
 30. The fuel cell stack as defined in claim 14, wherein thecathode comprises a catalyst layer in contact with the electrolytemembrane, and the catalyst layer of the second cell has a largerthickness in the stacking direction than the catalyst layer of the firstcell.
 31. The fuel cell stack as defined in claim 14, wherein thecathode comprises a catalyst layer in contact with the electrolytemembrane, and the catalyst layer of the second cell has a largerspecific surface than the catalyst layer of the first cell.
 32. A fuelcell stack which generates power by an electrochemical reaction betweenhydrogen and oxygen, comprising: a plurality of fuel cells stacked inseries, each of the fuel cells comprising an electrode and a gas passagefacing the electrode; wherein the fuel cells comprise a first celldisposed in a center position of the fuel cell stack in the stackingdirection of the fuel cells, and a second cell disposed in a positionother than the first cell with respect to the stacking direction of thefuel cells, and the gas passage of the second cell has a largercross-sectional area than the gas passage of the first cell.
 33. Thefuel cell stack as defined in claim 32, wherein a gas supply flowrate tothe gas passage of the second cell is set to be larger than a gas supplyflowrate to the gas passage of the first cell.